This application is based on Patent Application No. 11-312965 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an actuator using a displacement element such as a piezoelectric element or the like.
2. Description of the Related Art
In recent years, actuators have been proposed having two displacement elements such as a piezoelectric element or the like are arranged with their displacement directions set at a predetermined angle (e.g., 90xc2x0). In this actuator, an alternating current voltage signal having a specific phase difference drives each displacement element such that a drive member provided at the intersection point of the displacement elements moves in an elliptical path. This drive member abuts a driven member, and rotates or moves the driven member in a specific direction. Such an actuator is referred to as a truss-type actuator. The voltage or resulting displacement applied to the first piezoelectric element (displacement element) 10 and the second piezoelectric element (second displacement element) 10xe2x80x2 is shown in FIG. 16. When sine wave voltages of different phase, as shown in FIG. 16, are applied to the first piezoelectric element 10 and the second piezoelectric element 10xe2x80x2, the first piezoelectric element 10 and the second piezoelectric element 10xe2x80x2 are displaced in a sinusoidal wave-like fashion in accordance therewith. As a result, a tip member (drive member) 20 linked to the first piezoelectric element 10 and the second piezoelectric element 10xe2x80x2 moves elliptically (including circular movement).
When the frequency of the sine wave voltage applied to the first piezoelectric element 10 and the second piezoelectric element 10xe2x80x2 exceeds a predetermined value and the tip member 20 rotates at faster speed, the actuator itself cannot follow the displace the tip member 20 due to the force applied by the spring 41, and the tip member 20 is temporarily separated from the surface of the rotator 40 (intermittent contact). The tip member 20 moves in a direction opposite to a specified direction while the tip member 20 is separated from the surface of the rotor 40, so as to cause the rotor to rotate. This state is shown in FIGS. 17axcx9c17e. 
FIGS. 17a and 17e show the first piezoelectric element 10 and second piezoelectric element 10xe2x80x2 both extended, with the tip member 20 in contact with the surface of the rotor 40. FIG. 17b shows the first piezoelectric element 10 contracted and the second piezoelectric element 10xe2x80x2 extended, with the tip member 20 separated from the surface of the rotor 40. FIG. 17c shows the first piezoelectric element 10 and the second piezoelectric element 10xe2x80x2 both contracted, with the tip member 20 separated from the surface of the rotor 40. FIG. 17d shows the first piezoelectric element 10 extended and the second piezoelectric element 10xe2x80x2 contracted, with the tip member 20 in contact with the surface of the rotor 40 when the actuator overtakes the movement of the tip member 20. As can be understood from FIGS. 17axcx9c17e, the rotor 40 is rotated by the repeated contact with and separation from the surface of the rotor 40 by the tip member 20.
When the amount of displacement of the first piezoelectric element 10 and the second piezoelectric element 10xe2x80x2 is small and the amount of displacement of the tip member (drive member) 20 is less than several micrometers, a normal contact state arises between both the tip member 20 and the rotor 40 due to the elasticity of the materials of the tip member 20 and the rotor 40. In this instance, in essence disadvantages arise since the tip member 20 must separate from the surface of the rotor 40 and return in a specific direction so as to result in a reduction in speed of the movement of the rotor 40 due to the frictional force arising between both members, or the rotor 40 returns in the opposite direction in conjunction with the tip member 20 so as to reduce the output of the actuator. This phenomenon becomes more pronounced as the force exerted by the spring 41 increases.
In view of the aforesaid disadvantages, an object of the present invention is to provide an improved actuator.
A further object of the present invention is to provide an actuator having high drive efficiency by driving at optimum drive conditions even when the amount of displacement of the displacement element is small.
These and other objects are attained by one aspect of the present invention providing an actuator comprising:
a displacement element which produces a specific displacement;
a drive member connected to one end of the displacement element and which transfers the displacement of the displacement element to a driven member;
a stationary member which supports the other end of the displacement element;
a compression member which presses the drive member against the driven member; and
a drive circuit for driving the displacement element when the drive member and the driven member are in a state of intermittent contact, and under conditions near the condition of transition from the intermittent contact state to a normal contact state.
In the actuator, when the spring constant of the compression member is designated k1, the combined spring constant of the displacement element and the drive member is designated k2, the spring an constant of the driven member is designated k3, the amount of displacement of the displacement element is designated X0, and the compression force applied by the compression member is designated Nt, it is desirable that the following relationship is satisfied:
Nt=X0(1/(1/k2+1/k3)xe2x88x921/(1/k1+1/k2+1/k3))
It is further desirable that the displacement element is driven at a resonance frequency.